1. Field of the Invention
The present invention relates to a wireless communication system using array antennas, and more particularly to an apparatus and a method for transmitting and receiving packet data of an access terminal and a mobile terminal, which can improve the performance of a smart antenna system for processing signals in a frequency domain.
2. Description of the Related Art
According to rapid development in the communication technology, current wireless communication systems can provide not only typical voice communication services but also a packet data service capable of transmitting high capacity digital data. Mobile communication systems, which are currently being provided or researched in relation to the packet data service, include International Standard (IS)-2000 systems, Evolution Data Only (EV-DO) systems capable of supporting high speed packet data transmission, and Evolution of Data and Voice (EV-DV) systems capable of simultaneously supporting the voice transmission and the high speed packet data transmission, which are synchronous systems, and Universal Mobile Telecommunication Systems (UMTS), which are asynchronous systems.
The packet data services provided to mobile terminals can be briefly classified into services using 3rd generation mobile communication networks, such as CDMA 2000 1x, and services using wireless Local Area Network (LAN). The wireless LAN has a wide transmission bandwidth, which enables transmission and reception of packet data within short time. The wireless LAN provides a wireless broadband Internet service, all subscribers of which can share channels and efficiently use wireless channels.
In relation to the wireless LAN, the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standardization group has established the IEEE 802.16d standard for providing wireless broadband Internet service through a stationary terminal, which is a combination of the IEEE 802.16a and 802.16b standards, and is currently preparing the IEEE 802.16e standard for providing portable Internet service to a mobile terminal, which is an improvement of the IEEE 802.16d standard.
By the IEEE 802.16e based wireless broadband Internet service, it is possible to access the wireless Internet anytime and anywhere by a mobile terminal regardless of whether the mobile terminal is stopped or moving. A representative example of the wireless broadband Internet service is the service called “Wibro,” research of which is being rapidly progressed. Moreover, various communication schemes including Wi-Fi, Wi-Max, etc. are being researched for the wireless broadband Internet service. In the following description, each network which provides a wireless Internet service according to the IEEE 802.1x standard will be generally referred to as the “wireless LAN.”
In the environment in which attention to and demands for the wireless Internet is explosively increasing, an adaptive array antenna, which is also called an intelligent antenna or a smart antenna (hereinafter, referred to simply as “smart antenna”), is being researched as a solution for remarkably improving the quality and transmission speed of the wireless communication. The smart antenna system is a communication system in which a plurality of antenna elements are arranged in a particular pattern, so as to control the direction and beam width of the beams radiated from the antenna elements, thereby forming a directional beam toward a desired mobile terminal for data transmission and reception.
According to the basic principle of the smart antenna system, a transmission/reception beam is formed so that signals intended to be received are subjected to constructive interference and interference signals unintended to be received are subjected to destructive interference. By the smart antenna technology as described above, it is possible to regulate the interference signals at the receiver side and obtain the diversity gain and beam-forming gain, thereby remarkably improving the performance of the system.
The smart antenna technology has the following advantages. First, the transmission signals are not scattered but are collected at a desired location, so that the smart antenna technology can increase the signal gain. Therefore, it is possible to increase the area to be covered by each base station. Further, due to the increase in the signal gain, it is possible to reduce the power consumption of each terminal, which increases the battery use time. Second, signals in undesired directions are efficiently eliminated in the receiver. Therefore, the smart antenna technology can eliminate inference signals. Third, the smart antenna provides a spatial filtering effect, which can greatly reduce the influence of the multi-path.
The smart antenna system as described above can be applied not only to the 3G wireless mobile network or wireless LAN, but also to communication networks which use multi-wave transmission schemes, such as the Orthogonal Frequency Division Multiplexing (OFDM) scheme. The OFDM scheme is a representative multiple carrier transmission scheme, in which data are transmitted by a plurality of overlapping sub-carriers orthogonal to each other. According to the OFDM scheme, a serial input symbol sequence is converted into parallel signals, and the parallel signals or data are modulated with a plurality of mutually orthogonal sub-carriers and are then transmitted.
Hereinafter, a conventional smart antenna system will be described for an example of a wireless LAN system in which an Access Point (AP; hereinafter, referred to as “base station”) uses the smart antenna technology and a Mobile Terminal (MT) uses a single antenna. If a base station employing the smart antenna technology is completely compatible with a standard wireless LAN, it is possible to remarkably improve the performance of the wireless LAN system by replacing the conventional base station using a single antenna with a base station using a smart antenna, even when the existing mobile terminal is used as it is.
The base station using the smart antenna can transmit data with an omni-directional beam pattern by using a non-directional omni antenna according to a communication protocol. Usually, a fixed omni-directional beam pattern is used in order to transmit data with the omni-directional beam pattern. However, even when a fixed omni-directional beam pattern is actually generated by using a smart antenna, it is difficult to generate a beam pattern which is uniformly distributed over all directions, i.e. in 360 degrees. Therefore, when the conventional fixed omni-directional beam pattern is used, a user's mobile terminal located in a particular direction may experience trouble in communication. For example, FIG. 18 is a waveform graph of an omni-directional beam pattern generated by using a smart antenna implemented by four antenna elements.
Referring to FIG. 18, it is noted that, although an omni-directional beam pattern is generated, the generated omni-directional beam pattern is not evenly distributed in all directions of 360 degrees, because a cell of a base station includes places in which strong beams are formed and places in which weak beams are formed. Therefore, a problem may occur when communicating with a subscriber located in the direction of weak beam pattern.
FIG. 16 is a block diagram showing the structure of a transmitter in a downlink of a conventional smart antenna system.
Transmission (TX) data to be transmitted form a base station to a mobile terminal are mapped according to a predetermined mapping scheme by a mapper 1601, and are then multiplied by transmission weights, which are outputs of the multiplexer 1603, in multipliers 1605a˜1605d, respectively. For the transmission weights, TX beam forming weights are used when the communication protocol performs transmission beam formation using a smart antenna, and fixed TX omni weights are used when the communication protocol uses an omni-directional beam pattern using the omni antenna. The TX beam forming weights are preset to have predetermined weight values which enable a beam pattern formed by, for example, four antenna elements, to be most similar to the beam pattern of a typical omni antenna.
The transmission signals which have been multiplied by the transmission weights (TX beam forming weights or fixed TX omni weights) in the multipliers 1605a˜1605d are multiplied again in other multipliers 1609a˜1609d by calibration weights output from the multiplexer 1607 in order to compensate for the transfer function characteristic of the receiver side or transmission side of the system. For the calibration weights, predetermined TX beam forming calibration weights are used when the transmission weights are TX beam forming weights, and predetermined TX omni calibration weights are used when the transmission weights are fixed TX omni weights. The TX beam forming weights are obtained by using Reception (RX) beam forming weights. Because the RX beam forming weight includes an R element, which is a transfer function characteristic of the receiver side, the TX beam forming weight also includes the R element, which is a transfer function characteristic of the receiver side.
Therefore, in order to transmit data by using the TX beam forming weight, it is preferable to perform weight calibration in consideration of not only the T element, which is a transfer function characteristic of the transmission side, but also the R element, which is a transfer function characteristic of the receiver side. In conclusion, when the TX beam forming weight is used as the transmission weight, the TX beam forming calibration weight of R*/T must be used as the calibration weight, wherein * denotes complex conjugate. In contrast, when the fixed TX omni weight is used as the transmission weight, the calibration weight has a value having no relation to the transfer function characteristic of the receiver side and it is enough to compensate for only the transfer function characteristic of the transmission side.
The transmission signal which have been multiplied by the calibration weights in consideration of the transfer function characteristic of the system in the multipliers 1609a˜1609d are sequentially processed by Inverse Fast Fourier Transform (IFFT) units 1611a˜1611d for converting the frequency domain signals to time domain signals, Guard Interval (GI) inserters for inserting GIs to the OFDM data in order to prevent data loss due to inter-symbol interference, and TX RF units for RF processing 1615a˜1615d, and are then transmitted to a wireless network through antennas 1617a˜1617d. 
Hereinafter, a receiver of a mobile terminal for receiving the transmission signals of the base station in a downlink will be described.
FIG. 1 is a block diagram illustrating a structure of a receiver of a mobile terminal in a downlink of a conventional smart antenna system. It is assumed that a transmitter of the base station not shown in FIG. 1 performs, for example, signal processing in the frequency domain, and transmits data according to the OFDM transmission scheme by using multiple antenna elements.
A transmission signal of the base station, which has reached an antenna 101 of a receiver 100 of a mobile terminal after passing through a radio channel, is input to RX RF unit 103, is subjected to signal processing such as frequency down-conversion, and is then converted to a digital signal. From the digital signal obtained after the RX RF unit 103, a frequency offset is eliminated by a sub-carrier Frequency Offset estimation and compensation Unit (hereinafter, referred to as “FO”) 105.
The signal output from the FO 105 is input to a Fast Fourier Transform (FFT) unit 107 for converting a time domain signal to a frequency domain signal and an FFT window detector 109 for determining window setup of the FFT unit 107. For the window setup of the FFT unit 107, a reference point of a window and a window offset must be set in advance. To this end, the FFT window detector 109 detects an exact OFDM symbol boundary from the output of FO 105, and sets an FFT window with a margin as large as the FFT window offset with reference to the detected symbol boundary. Then the FFT unit 107 performs FFT according to the FFT window set by the FFT window detector 109.
The frequency domain incoming signal output from the FFT unit 107 is input to a frequency domain equalizer (FEQ) 111 for elimination of interference signals. A value estimated by the FEQ estimator 113 is used as the FEQ coefficient for the operation of FEQ 111. Further, the FEQ estimator 113 receives the signal output from the FFT unit 107 and a predetermined reference signal for estimation of an FEQ coefficient, estimates the FEQ coefficient by using the incoming signals, and then transfers the estimated FEQ coefficient to the FEQ 111. The reference signal may be, for example, a long preamble signal, which is a reference training pattern signal defined in the wireless LAN standard IEEE Std 802.11a-1999.
The output signal of the FEQ 111 passes through a Timing Offset estimator and compensator (hereinafter, referred to as “TO”) 115 for compensation of timing offset and then passes through a Residual Frequency Offset estimator and compensator (hereinafter, referred to as “RFO”) 117 for compensation of residual frequency offset. After the residual frequency offset is compensated by the RFO 117, the signal is demodulated by a demapper 119 according to a demodulation scheme corresponding to a predetermined modulation scheme such as Quadrature Phase Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (QAM), etc. and is then output as RX data. Then, channel decoding, etc. are performed in the stages after the demapper 119, so as to restore the original data.
In the receiver using the OFDM transmission scheme as described above, although an FFT window offset of about two to four samples is usually set, it is problematic that the phase of an incoming signal is rotated due to the influence of the FFT window offset, so as to degrade the reception performance. However, the performance degradation due to the phase rotation of the incoming signal becomes more severe when incoming beam formation is performed by a receiver of a base station as shown in FIG. 2.
FIG. 2 is a block diagram illustrating a structure of a receiver of a base station in an uplink of a conventional smart antenna system. The base station shown in FIG. 2 uses a plurality of array antennas. For convenience of description, it is assumed that the base station uses four antennas.
The signal transmitted from a mobile terminal of a subscriber is received by array antennas 201a˜201d of a base station receiver 200 through a radio channel. The signal received by the array antennas 201a˜201d is input to RX RF units 203a˜203d of corresponding RF chains for RF processing such as frequency down-conversion, and is then converted to digital signals. From the signals output from the RX RF units 203a˜203d, frequency offsets are eliminated by FOs 205a˜205d located in corresponding signal paths, respectively.
The signals output from FOs 205a˜205d are transferred to FFT units 207a˜207d on respective signal paths and input ports of an FFT window detector 209 for determining the window setup of the FFT units 207a˜207d. Meanwhile, in order to set a reference point of a window and a window offset in advance, the FFT window detector 209 detects an exact OFDM symbol boundary from the output of each of the FOs 205a˜205d, and sets FFT windows for the FFT units 207a˜207d with a margin as large as the FFT window offset with reference to the detected symbol boundary. Then, the FFT units 207a˜207d perform FFT according to the FFT windows set by the FFT window detector 209.
The frequency domain incoming signals output from the FFT units 207a˜207d and the RX beam forming weights generated by the RX beam forming weight calculator 215 are multiplied in the multipliers 211a˜211d, and the products of the multiplications are then added in the adder 213, so as to perform RX beam formation. The RX beam forming weight calculator 215 calculates an optimum RX beam forming weight for each antenna path by using a predetermined reference signal for FEQ coefficient estimation and an incoming signal through each antenna path output from the FFT units 207a˜207d. The reference signal may be, for example, a long preamble signal, which is a reference training pattern signal defined in the wireless LAN standard IEEE Std 802.11a-1999.
The incoming signal output from the adder 213 is input to an FEQ 217 and an FEQ estimator 219. The FEQ estimator 219 estimates an FEQ coefficient for the operation of the FEQ 217 by using the reference signal and the incoming signal output from the adder 213. The signal output from the FEQ 217 passes through a TO 221 and an RFO 223, while the signal is compensated for a timing offset and a residual frequency offset. After the compensation by the TO 221 and the RFO 223, the signal is demodulated by a demapper 225 according to a demodulation scheme corresponding to a predetermined modulation scheme such as QPSK, 16 QAM, etc. and is then output as RX data.
The conventional smart antenna systems as shown in FIGS. 1 and 2 have the following problems.
A smart antenna system using an OFDM communication scheme proper for the frequency domain signal processing necessarily requires a process of detecting an exact OFDM symbol, setting an FFT window according to a result of the symbol detection, and performing FFT. However, in a usual OFDM system, in order to improve the performance, FFT is performed by using an FFT offset of about two to four samples, instead of using an FFT window set based on the exact OFDM symbol boundary.
However, after the FFT, the phase of the incoming signal rotates a predetermined angle due to the influence of the FFT window offset. Such a phase rotation may have an effect on the calculation of RX beam forming weight at the receiver side of the smart antenna system, thereby degrading the performance of the system. Further, when a TX beam forming weight is obtained by using the RX beam forming weight, the performance of the TX beam forming weight may also be degraded.
Hereinafter, the influence on the RX beam forming weights by the FFT window offsets will be described with reference to the results of the following experiments.
FIGS. 3A through 3C are waveform graphs for illustrating a reference signal for FEQ coefficient estimation when the FFT window offset is zero, an incoming signal, and a weight signal for RX beam formation in the receiver of FIG. 2, respectively.
FIG. 3A illustrates a reference signal used for FEQ coefficient estimation when the array antennas include four antenna elements and the FFT window offset is 0 sample, and FIG. 3B illustrates an incoming signal corresponding to the reference signal of FIG. 3A. Further, FIG. 3C illustrates an RX beam forming weight signal which is a resultant signal obtained through calculation using the reference signal and the incoming signal. It is noted that no phase rotation occurs in the RX beam forming weight signal as shown in FIG. 3C when the FFT window offset is 0. However, the OFDM system cannot avoid performance degradation when no FFT window offset is given as shown in FIGS. 3A through 3C.
In general, an FFT window offset is given by the following reasons. First, the performance degradation increases when no FFT window offset is given and FFT is performed by putting a point after the exact FFT window start point as the FFT window start point. Therefore, the first reason is to arrange a margin by an FFT window offset based on the exact FFT window start point. Second, it is helpful for performance improvement to also use signals before a strong signal path which is mainly detected at the time of FFT window detection when there exist multi-paths for OFDM symbol reception.
However, even when a proper FFT window offset is set for the performance improvement of the OFDM system, the following problems occur.
FIGS. 4A through 4C are waveform graphs for illustrating a reference signal for FEQ coefficient estimation, an incoming signal, and a weight signal for RX beam formation, respectively, when the FFT window offset is one in the receiver of FIG. 2.
FIG. 4A illustrates a reference signal used for FEQ coefficient estimation when the FFT window offset is 0 sample likewise in FIG. 3A, and FIG. 4B illustrates a waveform of an incoming signal output after being phase-rotated when an FFT window offset of 1 is set to the signal waveform FIG. 3A. Further, FIG. 4C illustrates an RX beam forming weight signal which is a resultant signal obtained through calculation using the reference signal and the incoming signal. It is noted that the phase rotation in proportion to the window offset occurs in the RX beam forming weight signal when the FFT window offset is set as described above. The phase-rotated RX beam forming weight degrades the performance of the smart antenna system. Further, the TX beam forming weight obtained by using the RX beam forming weight is also influenced by the phase rotation, and the performance of the TX beam forming weight is thus also degraded.
As described above, in the case of a conventional smart antenna system, even when a fixed omni-directional beam pattern is actually generated by using the smart antenna, it is difficult to generate a beam pattern uniformly distributed over all directions of 360 degrees, and a mobile terminal located in a particular direction may experience trouble in communication. Further, the phase of the incoming signal after FFT is rotated due to the FFT window offset at the receiver side, so as to degrade the performance of the smart antenna system. Further, although the base station performs RX and TX beam formation by employing the smart antenna for improvement in the transmission and reception performance, a subscriber's mobile terminal which has a single antenna performs passive operation simply depending on the base station.